Recovery of xylenes



Aug. 6, 1946. R B, GREENBURG 2,405,300

RECOVERY OF XYLENES Filed Jan. '7, 1944 Condenser www ATTORNEY.

Patented Aug. 6, 1946 Vuru'rrsl) `sfm'nt nEcovEaY or xYLENEs Richard B. Greenburg, Melrose or to Allied York, N. Y.,

ark, Pa., assign- Chemical & Dye Corporation, New a corporation of New York Application January "l, 1944, Serial No. 517,335 t Claims. (Cl. 2055-42) term as thus used includes the three xylenes,

ortho, metaand para-xylene, and ethyl benzene.

Numerous hydrocarbon oils are known which contain xylenes in varying proportions. For example, gasoline fractions obtained by the distillation of certain types of petroleum frequently contain substantial proportions of xylene, al-

.though mainly consisting of other hydrocarbons.

Oils of petroleum origin having a considerable content of aromatics, including xylene, may be treated by well known selective solvent processes to produce fractions rich in aromatics; for example extraction of suitable fractions of such petroleum oils with sulfur dioxide may yield fractions of increased xylene content. In such cases xylene is accompanied by non-aromatic oils which may be largely paraflinic, naphthenic or oleiinic in character. A considerable portion of these non-aromatic oils cannot be separated from the xylene by direct fractional distillation because of the closeness of their boiling points to that of xylene or because they form constant boiling mixtures with xylene. Also synthetic hydrocarbon liquid and gas mixtures produced by various catalytic processes may contain xylene which is accompanied by similar difilcultly separable constituents.

While ordinarily xylene may be separated by direct fractional distillation from light oils pro` duced by the gasification of coal, in some cases the xylene is accompanied by diflicultly separable non-aromatic oils of the same general character as described, owing to carbonization conditions, type of coal used or other special circumstances.

'This is particularly true of drip oils and oil fractions recovered therefrom containing ortho-xylene and like-boiling, non-aromatic hydrocarbons, e. g. naphthenes, parafiins and oleiins.

By "drip oil I refer to those condensates from carburetted water gas such as the condensates formed in the gas distributing lines or holders or the light oil which may be condensed in cooling the carburetted water gas lafter removal of tar. A common characteristic of drip oils is that in addition to aromatic hydrocarbons, they contain a substantial proportion of like-boiling, non-arothe appended claims thev matic hydrocarbons including oleiins and paraffins or naphthenes,

By fractional distillation of these oils containing xylene, fractions of increased xylene content may be obtained. These xylene fractions, however, will still contain large amounts of the other constituents of the oil having boiling points in the neighborhood of the boiling point of xylene or forming mixtures of constant boiling points in the range of temperatures at which Xylene distills from the oil. Accordingly, recovery of xylene of a relatively high purity from these sources presents a diillcult problem of great industrial importance.

The fact that the oils containing xylene have generally a relatively low content of xylene makes it particularly diiilcult to recover by fractional distillation or selective solvent extraction or a combination of both of these procedures a xylene product of a high degree of purity. For example, drip oils may contain about 5% ortho-xylene. Distillation of a drip oil with efficient rectification of the vapors may b e employed to recover a fraction in which the ortho-xylene is concentrated. However, even with efficient rectification a fraction containing more than about to ortho-xylene can rarely be obtained. Olefins together with paraflins and usually some naphthenes and traces of metaand para-xylene, make up the remaining 15% to 20% of the orthoxylene fraction.` The non-aromatic hydrocarbons individually may have boiling points ranging from considerably below' to above the boiling point of ortho-xylene, e. g. their boiling points may range from to 155 C., whereas orthoxylene boils at about 144 C. Nevertheless, these non-aromatics cannot be 'separated from the ortho-xylene by fractional distillation of their mixtures, e. g. drip cils and cracked petroleum oils, under practical conditions.l Apparently in the mixture the various hydrocarbons form azeotropes with each other and this prevents their separation by fractional distillation. Such hydrocarbons, those which cannot be separated by fractional distillation or are difilcultly separable by fractional distillation under practicable conditions for distilling oils, are referred to as like-boiling hydrocarbons. Chemical treatment of oils containing ortho-xylene and like-boiling hydrocarbons may be used to recover pure orthoxylene therefrom, but such known methods are cumbersome and expensive.

It is, therefore, an object of this invention to provide a process for azeotropically distilling the xylene-containing oil in the presence of an azeo- -3 v ;ropic agent which is particularly eiTectiv-e for separating from the xylene klike-boiling, nonaromatic compounds contained ln the oil to obtain a commercially pure xylene product.

It is a further object of the invention to provide a process for the separation of xylene from fractions of petroleum oil or drip oil containing xylene and a mixture of other hydrocarbons -of similar boiling range.

I have discovered that ethylene diamine forms azeotropes with the xylenes and also with the non-xylene hydrocarbon constituents present in the oil and distilling therefrom in the range of temperatures at which the xylene distills from the oil. The xylene azeotropes, however, boil at a substantially highertemperature than the non- `xylene ethylene diamine azeotropes, permitting separation of the xylene. It is significant to note that the spread between the boiling point of the xylene-ethylene diamine azeotropes and the point at which the non-xylene-ethylene diamine azeotropes begin to distill over is greater in the case of the ethylene diamine than in the case of many of the numerous other azeotropic agents tested in the extensive development work which led up to the present invention.

The presence of substantial amounts of water in the complex mixture constituted of xylene, hydrocarbons of parafilnic character distilling at temperatures close to that of xylene, other close boiling hydrocarbons which may be of naphthenic or oleflnic character, and ethylene diamine, should lbe avoided. Even the presence of 1% of water is objectionable in that it involves some loss of xylene. Accordingly, the preferred embodiment of the invention involves the use of substantially anhydrous ethylene diamine as azeotropic agent, e. g. the water present being limited to no more than 5% by weight of the ethylene diamine. More water may be present at a sacrifice in the eiliciency of the process but in no event should it exceed by weight of the ethylene diamine. The expression ethylene diamine substantially free of water is used in this specification and appended claims to define an anhydrous ethylene diamine or an ethylene diamine containing up to but no more than 10% water.

In carrying out the distillation of the hydrocarbon mixture containing xylene, close boiling non-aromatic hydrocarbons, and ethylene diamine to produce relatively pure xylene, the distillation is conducted with rectiiication of the evolved vapors While maintaining at a point in the rectication of the vapors a temperature not above 114 C. By supplying to the distillation adequate amounts of ethylene diamine, this control temperature may be maintained and the distillation continued until the unvaporized residue contains at least 95 parts by weight of xylene for every 5 parts by weight of non-aromatic hydrocar-bons of similar boiling range to that of xylene.'

If the control temperature should tend to rise above 114 C., this is an indication that the amount of ethylene diamine is inadequate and more of this material should be introduced. The closer to 114 C. the control temperature is maintained the purer the xylene product. By operating in accordance with these conditions I have found it possible to produce from'xylene fractions boiling Within the range 125 to 150 C. substantially 100% pure xylene; i. e., to eifect complete separation from xylene of the hydrocarbons of similar boiling range to that of xylene by continuing the distillation until only xylene and ethylene diamine are left in the still or distill 4 over. the ethylene diamine being then separated from the xylene in any desired manner. for ex' ample, by washing the mixture with dilute hydrochloric acid followed by a water wash of the xylene to remove the acid and dehydration of the washed xylene.I

The ratio of ethylene diamine substantially free of water to the xylene fraction which may be employed in carrying out this invention depends upon the amount and nature of the hydrocarbon impurities in the xylene fraction, the purity desired in the xylene residue from the distillation, the proportion of xylene in the original xylene fraction which is to be recovered in the residue, the procedure used for distillation of the ethylene diamine-xylene mixture, and the equipment employed. The quantity of ethylene diamine used in the distillation of hydrocarbons from a given quantity of xylene fraction should be in excess oi' that which will form azeotroplc mixtures with the non-xylene hydrocarbons which are to be vaporized and taken over into the distillate. By returning most or all of the ethylene diamine collected from the distillate to the still, for example, as 'reux liquid, the total amount of ethylene diamine necessary to eilect the separation of the non-xylene constituents and the xylene will be materially reduced. If desired, fresh ethylene diamine may be introducedinto the xylene fraction being distilled during the course of the distillation; also, the ethylene diamine separated from the distillate may be returned continuously or periodically to the still or rectification column while the distillation of the xylene fraction is progressing.

Regulation of the temperature in the rectliication of the vapors to meet the aforesaid conditions is accomplished by maintaining an adequate quantity of the ethylene diamine substantially free of water in the still or rectification column or both during the distillation of the iwlene fraction to selectively carry over through the rectification treatment the vapors of the non aromatic hydrocarbons to be removed from the xylene. Some of the xylene may also be carried over, but as long as the temperature of 114 C. in the rectication zone is not exceeded, the undistilled residue will be enriched in xylene, and by continuing the distillation under the aforesaid conditions separation of the non-aromatic hydrocarbons of similar boiling range to that of xylene from the xylene will be accomplished` As the distillation proceeds temperature readings are taken of the temperature at the top of the rectification column and, by supplying additional azeotropic agent when required to prevent this temperature from rising above 114 C. an adequate amount of ethylene diamine substantially free of water will be present during the separation of the non-aromatic hydrocarbons from the xylene fraction. It is not necessary that this point of control temperature be at the top of the column, although this is a satisfactory point for determining this temperature in the equipment used for fractionating the vapors and condensing vthe fractionated vapors for effective use of the rectification column. One skilled in the distillation art will recognize suitable points for maintaining this control temperature in any specific apparatus according to well known distillation principles.

The distillation preferably is carried out at atmospheric pressure; it may, however, be carried out under vacuum or under superatmosphericfI pressure. If carried out under vacuum or superdistill the oil to recover therefrom an same temperature as hydrocarbons may be obtained by atmospheric pressure the control temperature is modified accordingly so that it corresponds tothe boiling point of the xylene-ethylene diamine azeotrope under the changed pressure. The temperatures as given in this specification and the appended claims are corrected temperatures for 1 atmosphere pressure (760 mm. of mercury).

In practicing this invention in the treatment of an oil such as has been described above, containing xylene and other hydrocarbons, particularly when the xylene concentration of the oil is low or the oil is one containing materials of wide boiling range, it is preferred to iirst fractionally enriched xylene fraction which contains in addition to xylene other hydrocarbons which distill at the the xylene. Although xylene fractions having an end boiling point above the boiling point of xylene (e. g. a boiling point up to 150 C.) may be azeotropically distilled in the manner hereinafter described, I prefer the xylene fraction recovered in the preliminary distillation step be one having a maximum boiling point substantially corresponding to the boiling point of ortho-xylene, i. e. 145 C. Further, it is preferred that the xylene fraction recovered by the preliminary distillation of the crude xylene oil be so cut as to exclude therefrom the forerunnings which do not contain substantial proportions of xylene, for example, to exclude any materials distilling below 125 C. and preferably below 135 C.

A xylene fraction such as may be obtained by the preliminary distillation hereinabove described, e. g. a fraction boiling Within the range 125 to 150 C., preferably 135 to 145 C., and which may contain parafiins, naphthenes or olens, is subjected to a second distillation and rectification of the vapors in the presence of ethylene diamine substantially free of water. When the hydrocarbon mixture is subjected to fractional distillation with rectification of the vapors under conditions such that a temperature not exceeding 114 C. is maintained at a point inthe rectification of the vapors, the azeotropes of non-aromatic hydrocarbons and ethylene diamine may be distilled oi from the mixture to leave a residue containing at least 95 parts by weight of xylene to every 5 parts by weight of like-boiling non-aromatic hydrocarbons originally present in the xylene fraction and not separable therefrom by direct fractional distillation in the absence of the azeotropic agent.

After the distillation has been carried to the point where the residue contains xylene of a desired purity with respect to non-aromatic hydrocarbons of similar boiling 'range to xylene, the distillation may be stopped and the residue withdrawn from the still, This residue ordinarily will contain xylene and other hydrocarbons in the proportion of 95 or more parts xylene to 5 or less parts of the total hydrocarbons other than xylene. By suitably limiting the top boiling point of the xylene fraction which is azeotropically distilled a residue of the azeotropic distillation may be produced of desired purity of its yhydrocarbon content greater than 95 parts xylene to 5 parts non-aromatic hydrocarbons. A residue containing the xylene substantially free of non-aromatic distilling a xylene fraction having a top boiling point not higher than 145 C. in the presence of ethylene diamine in amount such that a temperature of substantially 11B-114 C. is reached but is not lexceeded at a point in the rectification of the and the last fraction residue was washed with 10% distilled vapors. The residue of the azeotropic distillation may be purified further as desired to remove any ethylene diamine which it contains and to remove any other impurities present. When the distillation is carried to the point at which all of the ethylene diamine hasl been distilled out of the residue and the xylene constitutes substantially 99% or more of the total hydrocarbon content of the residue, the residue may be given a conventional treatment, for example, treatment with sulfuric acid and redistillation, to obtain aproduct which is suitable for marketing as a nitrationgrade xylene of particularly high purity.

Instead of withdrawing the xylene residue from the still, the distillation may be continued and the xylene distilled over and separately collected from the distillate containing the hydrocarbons from which the xylene residue previously had been separated by azeotropic distillation. If ethylene diamine is present during the distillation and collection of the xylene, the ethylene diamine may be distilled out in the form of an azeotrope of xylene. The xylene may be separated in any suitable manner from this azeotrope, for example, by treatment with HCl of 10% concentration which will remove the ethylene diamine from the xylene.

The following example is illustrative of the use of my invention for the recovery of xylene from oils containing it together with like-boiling, nonaromatic hydrocarbons: I

An apparatus suitable for carrying out the process of this example is, diagrammatically illustrated in the accompanyingdrawing. It comprises a still l with heater 2 and rectification column 3 of conventional design. A condenser 4 is provided to which the vapors from the top of the column pass and in which they are cooled and condensed. A pipe 5 returns a portion of the condensate from the condenser to the top of the rectication column to serve as reilux in the column. lA second pipe 6 is provided for drawing 01T a portion of the condensate as product. Pipes 1 and 8 are provided for introduction of liquids into still I and a pipe 9 for withdrawal of residue from this still. f

By fractional distillation of an aromatic petroleum oil a cut was taken distilling over at 127 to 150 C. in which the xylene was concentrated. This xylene fraction contained 58% by volume xylene. To every 100 volumes of the xylene cut introduced into still I, 100 volumes of an ethylene diamine substantially free from water ethylene diamine and 5% Water) was added and the mixture distilled with rectification of the vapors in a conventional rectication'column 3 supplied with reflux obtained by returning to the column through pipe 5 a part ofthe condensate from the vapors leaving the top of the column. Another part of this condensate was drawn olli as overhead distillate through pipe 6.

" Distillation started with the vapors at the top of the column at a temperature of 103.6 C. This temperature rose during the distillation to 114 C.

distilled over at that temby volume xylene. The hydrochloric acid solution to remove ethylene diamine and the oil separated from the solution was found to contain 98% by volume xylene.

It is, of course, obvious preliminary distillation of a crude xylene oil to obtain a xylene perature contained 95% of the petroleum oil as the xylene, and, therefore, are not separable from the xylene by direct fractional distillation. As used in this specincation and the appended claims, the term xylene fraction refers to all such xylene-containing oils, yWhether produced by fractional distillation of xylene-containing materials or by any other means. As has been pointed out above, these xylene fractions in general contain substantial quantities (15% or more) o f non-aromatic hydrocarbons which it is particularly difficult, and in many cases impossible, to separate from the xylene by any practicable method of fractional distillation or solvent ex traction. The process of my invention is particularly valuable as a means for recovering relatively pure xylene from such fractions. Y

I claim: Y

1. A process for the recovery of xylene from a complex hydrocarbon fraction having a top boiling point no higher than 150 C. containing xylene and non-aromatic hydrocarbons which distill from said fraction in the same temperature range as the xylene distills therefrom, which comprises azeotropically distillng said fraction and rectifying the vapors evolved therefrom in the presence of ethylene diamine substantially free from water, thereby removing said nonaromatlc hydrocarbons as azeotropic distillate with ethylene diamine, and maintaining the presence of ethylene diamine during said distillation .until the residue of distillation contains at least 95 parts by weight of xylene to every 5 parts by weight of non-aromatic hydrocarbons which distill in the absence of ethylene diamine in the same temperature range as the xylene.

2. A process for recovering xylene from a xylene fraction boiling within the range of 125 to 150 C. containing xylene and non-aromatic hydrocarbons which distill from the xylene fraction in the same temperature range as the xylene distills therefrom, which process comprises, distilling said xylene fraction and rectifying the vapors evolved therefrom in the presence of ethylene diamine substantially free from water while maintaining at a point in the rectincation oi' the vapors a temperature not above 114 C. by maintaining the presence of -said ethylene diamine at said point, whereby said non-aromatic hydrocarbons are removed as azeotropic distillate with ethylene diamine, and continuing the saidv distillation until the unvaporized residue of the distillation contains at least parts by weight of xylene to each 5 parts by weight of nonaromatic hydrocarbons which distill from said fraction in theabsence of ethylene diamine in the same temperature range as the xylene.

3. A process for the recovery of substantially pure xylene from a complex hydrocarbon fraction boiling within the range of to 145 C. containing xylene and at least 15% of non-aromatic hydrocarbons which distill from said fraction in the same temperature range as the xylene distills therefrom, which comprises distilling said fraction and rectifying the vapors evolved therefrom in the prence of ethylene diamine substantially free from water while maintaining at a point in the rectincation of the vapors a temperature not above .114 C. by maintaining the presence of said ethylene diamine at said point and continuing the said distillation until substantially all of the non-aromatic hydrocarbons present in said fraction are removed as an azeotropc mixture with ethylene diamine, leaving a residue of the distillation containing the xylene substantially free of non-aromatic hydrocarbons.

4. A process for the recovery of substantially pure xylene from a complex hydrocarbon fraction having a top boiling point not higher than C. containing xylene and non-aromatic hydrocarbons including parainic hydrocarbons and which may also include naphthenicand olenic hydrocarbons, said non-aromatic hydrocarbons normally distilling from said fraction at about the same temperature at which the xylene distills therefrom, which process comprises distilling said fraction and rectifying the vapors evolved therefrom in the presence of ethylene diamine substantially free from water present in amount such that a temperature of substantially 113-114 C. is reached but is not exceeded at a point in the rectification zone, and continuing the distillation in the presence of ethylene diamine until substantially all of the non-aromatic hydrocarbons in said fraction are removed as azeotropic distillate with the ethylene diamine, leaving a residue containing the xylene substantially free of non-aromatic hydrocarbons.

RICHARD B. GREENBURG. 

